The present invention relates in general to instruments which are used to examine the interior of the eye and in particular to ophthalmoscopes, as they are often called.
Because an observation of the patient's retina is a prerequisite of a diagnosis, ophthalmoscopes play an important role in a clinical examination. However, since the time when ophthalmoscopy became part of every clinical examination, ophthalmologists have been anxious to improve the performance of ophthalmoscopes, and in particular they have been anxious to improve the field of view and the observable field that can be obtained with an ophthalmoscope. In connection with the foregoing, the field of view is the area of the fundus covered by the observer's eye in any given direction of observation, i.e., the area of the fundus that is seen as one view without scanning, and the observable field is the area of the fundus limited by a boundary beyond which the ophthalmoscope is unable to reach, i.e., the total area of the fundus that can be seen in several views by scanning. The effort that has been made to increase both of these fields has been continuous and somewhat successful.
Ophthalmoscopes are classified into two types or groups--direct ophthalmoscopes and indirect ophthalmoscopes. Direct ophthalmoscopes enable a field of view not larger than 12.degree. of arc, which corresponds to a relatively small area of the fundus (4 mm. in diameter) and only enable an observable field hardly reaching the equator. Direct ophthalmoscopes, however, enable a 15-power magnification of the field of view. Thus, although the direct ophthalmoscope only enables a field of view of barely 2 to 3 disk diameters, this instrument is employed when high magnification of that area is desired. In this regard, prior art indirect ophthalmoscopes only enable a three-power magnification of the field of view.
Because of the inability of direct ophthalmoscopes to provide a large field of view, indirect ophthalmoscopes are widely in use. One reason for the use of indirect ophthalmoscopes, besides the ability of this instrument to explore the peripheral retina more efficiently, is their larger field in view. In this regard, the indirect ophthalmoscope extends the field of view that can be provided to about 35.degree. of arc, that is 8 to 10 disk diameters, and by moving the instrument to scan the patient's eye the observable field that results includes the ora serrata.
The parameter limiting the field of view in the indirect ophthalmoscope is the numerical aperture of the condensing lens. The recent improvement of the condensing lens by increasing its numerical aperture up to F/1 permits an increase in the field of view to approximately to 45.degree. with a +20D lens. The numerical aperture F/1 is the approximate limit of the optical performance possible. A slight increase (55.degree.) can still be gained with the +30D lens at the expense of loosing 1/3 magnification.
A distinct disadvantage of all known ophthalmoscopes is that they do not permit a single view of the fundus from the posterior pole to the equator. In order to obtain such a view of the fundus to make accurate determinations of the diseased state, an ophthalmoscopist must scan this portion of the fundus and draw a map of what he sees as he views one area of the retina and then the next. A hand-drawn or photo map is the only image of the area of the retina from the posterior pole to the equator that the ophthalmoscopist has to work with.
The ideal ophthalmoscope or the ideal fundus camera should give a picture of the whole retina from the posterior pole to the ora serrata. Such a picture would eliminate the time consuming and inaccurate mapping of the fundus which is tiring for both the patient and the ophthalmologist. In accordance with one embodiment of the present invention, which includes clad fibers, such mapping is significantly reduced because the instrument enables a single view of the fundus from the posterior pole to the equator. In another embodiment of the invention, which includes "selfoc" fibers, a single view of the entire retina is possible.
Thus, the present invention is a wide-angle ophthalmoscope which includes fiber optic light guides for illuminating the retina. The light guides function in conjunction with a lens to provide a wide-angle view of the field which is uniformly illuminated by the fiber optic light guides.
An illuminator which includes fiber optics to illuminate the retina is disclosed in French Pat. No. 1.583.436 by Raymond Andre Dudragne.
The illuminator disclosed in this French patent, however, is not considered suitable because it provides only a very restricted field of observation and can produce reflections of the illuminating light which can blur the image of the retina. In addition, French Pat. No. 1.583.436 suffers from a serious deficiency in that this patent does not disclose the proper location of the ring of illuminating fibers, nor does it disclose the importance of the angle of inclination of the illuminating fibers. Furthermore, the French patent does not disclose the importance of wide-angle fibers.
In connection with the foregoing, an important aspect of the present invention is the location of the illuminating circular bundle of fibers, the inclination of the fibers at the contact point with the cornea and the numerical aperture of the fibers. These points are amplified below.